The present invention relates to a system for controlling an electromagnetic clutch for an automotive engine, and more particularly to a system for controlling clutch current at engagement of the clutch.
An electromagnetic powder clutch as a type of electromagnetic clutch for a motor vehicle is known, for example in U.S. Pat. No. 4,494,641. The electromagnetic powder clutch comprises an annular drive member secured to a crankshaft of an engine, a magnetizing coil provided in the drive memeber, a driven member secured to an input shaft of a transmission and spaced adjacent to the drive member by a small gap and magnetic powder provided in a chamber in the clutch. Change speed gears in the transmission are changed by operating a selector lever. The selector lever is provided with a switch for the circuit of the magnetizing coil. When the selector lever is gripped by the operator's hand, the switch is opened to cut off the clutch current. Accordingly, a change gear operation of the transmission can be done. When the selector lever is shifted to the gear engaging position and released from the hand, the switch is closed so that electric current flows through the magnetizing coil to magnetize the drive member. As engine speed increases in accordance with the depression of an accelerator pedal, the clutch current applied to the coil increases with increase of repetition frequency of ignition pulses. The magnetic powder aggregates in the gap between the drive member and the driven member, so that the driven member is coupled to the drive member. More particularly, the clutch current passing through the magnetizing coil progressively increases according to an increase of the frequency of the ignition pulses, while the clutch slips between the drive member and driven member and gradually engages until the clutch current increases to rated current. Thus, the motor vehicle may be smoothly started by depressing the accelerator pedal without operating a clutch pedal.
FIG. 9 shows the relationship between engine speed and clutch torque. The clutch torque increases exponentially with an increase of engine speed. However, it is desirable to vary the speed of the engagement of the clutch in accordance with the speed of increase of the engine speed, in order to reduce shock at engagement of the clutch.